1. Field of the Invention
The present invention is related to data communications. In particular, the present invention is related to a method for resolving a request for information in a multiprotocol internetwork environment operating over Non-Broadcast Multiple Access (NBMA) subnetworks.
2. Description of the Related Art
The Next Hop Resolution Protocol (NHRP) is described in Luciani, J., Katz, D., Piscitello, D., Cole, B., and Doraswamy, N., “NBMA Next Hop Resolution Protocol (NHRP)”, IETF RFC 2332, April, 1998. With reference to FIG. 1, the NHRP allows a source station 110 (a node, host, switch, router, etc.) in a multiprotocol internetwork 100 to communicate with a destination station 130 over a Non-Broadcast, Multiple Access (NBMA) subnetwork 105. The NHRP provides source station 110 with the capability to determine the NBMA address of the NBMA next hop toward the destination station, through the exchange of NHRP resolution requests and resolution replies. As pointed out in RFC 2332, the NBMA subnetwork may be non-broadcast because it does not support broadcasting (e.g., an X.25 or ATM subnetwork) or broadcasting is not possible, for example, in the case of a Local Area Network (LAN) that supports a large number of stations. In FIG. 1, the NBMA next hop toward the destination station is router 120 since it is the closest router to destination station 130 and provides egress from the NBMA subnetwork. It should be noted that terms used herein to describe the present invention, such as internetwork layer, server, client, and station, are to be interpreted in a manner consistent with the definition and use of such terms as provided in RFC 2332.
In accordance with the NHRP, Next Hop Servers (NHSs) are provided in the NBMA subnetwork which are capable of responding to NHRP resolution requests. In FIG. 1, egress router 125 also functions as a NHS, and serves one or more destination stations, such as destination station 130. Likewise, ingress router 115 must function as a NHS for station 110.
An NHS builds and maintains a data structure that contains internetwork layer address (e.g., an Internet Protocol address) to NBMA subnetwork layer address resolution information. The table may be built and managed in accordance with techniques known to those of ordinary skill in the related arts. For example, a station may send a NHRP registration request to a NHS serving the station. The NHRP registration request contains internetwork layer address to NBMA subnetwork layer address resolution information that is then stored in the table maintained by the NHS.
A station that is a client of the NHRP service is known as a NHRP Client, or simply NHC. The NHS with which a NHC communicates to provide NBMA next hop information is the serving NHS for the NHC. In FIG. 1, NHS 115 serves station (NHC) 110, and NHS 125 serves station (NHC) 130 in most cases. For a serving NHS to supply address resolution information to a NHC, a continuous link of NHSs must exist along a path in the NBMA subnetwork between the NHC making the NHRP resolution request and the destination NHC, e.g., NHSs 115, 120 and 125. In accordance with RFC 2332, the last NHS along the path within the NBMA subnetwork is the serving NHS. That is, NHRP resolution requests are not forwarded to destination station/NHCs but are processed by the serving NHS. However, each NHC also maintains a table of intemetwork layer address to NBMA address resolution information that it obtains from NHRP resolution replies, manual configuration, or through mechanisms outside the scope of the NHRP. Destination NHCs may be constrained on resource (e.g., SAR VPI/VCIs), and there is no existing mechanism to communicate that fact to the serving NHS which would normally reply to a resolution request on behalf of the NHC. Thus, the only way that a source station would find that out would be to attempt a connection setup and fail which is time consuming and resource intensive. By allowing the destination NHC to reply for itself, since it is in the best situation to know whether it has resource enough for the connection, signaling efficiency is gained since no additional connection setup attempt will be tried by the source station. Moreover, in the situation where multiple NHCs register with a given serving NHS for the same set of NBMA subnetwork addresses (e.g., for the same set of ATM attached servers/services), the NHS may perform service load balancing by forwarding the resolution request to a particular NHC and if that NHC then NAKs the resolution request, the serving NHS may offer the request to another NHC which is not currently busy. Further the NHS may choose to offer the request in some scheduled fashion (e.g., round robining) to each of the appropriate NHCs in turn.